A teenage boy who has admitted murdering a 63-year-old Callington woman has testified in the Supreme Court trial of his co-accused, saying he had thought about killing someone since he was in year one.

Pirjo Kemppainen was stabbed and bludgeoned to death at her home in 2010.

Two 14-year-old boys were charged with her murder, and one of the boys who has pleaded guilty has taken the stand as a star prosecution witness.

When asked in court how long he had thought about killing someone, the boy replied: "Since about year one."

The boy said he had spoken to his co-accused about the idea of killing someone, and the accused boy had agreed to help.

He said the pair then spoke about killing someone every weekend when the accused boy would sleepover at his house.

"I said something like it would be cool to kill someone and that," he told the court.

He said the accused boy had picked out the victim about a week before the murder, and had pointed at her house as they walked past saying she lived alone and was an easy target.

He said the pair practised stabbing a mattress because it was the closest thing they had to human skin.

The court heard, on the night of the murder, he had told the accused boy "tonight's the night" and the pair then planned how they would kill the woman.

He said the pair then continued playing on his Xbox and discussed the murder further, before going to the victim's house armed with knives.

He said they knocked on the door and the accused boy asked Ms Kemppainen if she had seen a lost dog.

He said as they left they argued about who was going to kill her, then drank more beer and returned.

When asked about his mood at the time, he said he was angry and under stress and had been suspended from school that day for fighting.

"I was in a pretty bad mood. I was very angry for that month getting wound up at school and that, the pressure started to build up and I just got stressed and angry and that," he said.

The Director of Public Prosecutions (DPP) Adam Kimber asked the boy why he killed the lady, to which he replied: "I don't know".